The present invention relates generally to light controlling systems. In particular, the present invention is directed to improved electronic flash apparatus.
Electronic flash apparatus is known in the art in which the flash of light produced by a flash tube is automatically terminated after a predetermined total quantity of light has been received from the scene being illuminated. In one particular type of electronic flash apparatus, a flash terminating switch is connected in series with the flash tube. When a light flash is to be produced, both the flash tube and the flash terminating switch are switched to a conductive state. When an exposure control circuit has received the predetermined quantity of light, the flash termination switch is switched into a non-conductive state, thereby terminating the flash.
One specific type of electronic flash apparatus of this general type is shown in FIG. 2 of U.S. Pat. No. 3,727,100 by Kuraishi et al. and in U.S. Pat. No. 3,809,954 by Engelstatter. In this specific type of electronic flash apparatus, a flash trigger switch (generally a silicon controlled rectifier (SCR)) is used to trigger the flash tube and the flash termination switch. Contacts and a capacitor are connected to the control electrode (gate) of the flash trigger switch. When the contacts close, the capacitor discharges, thereby switching the flash trigger switch to a conductive state.
The flash trigger switch is connected in series with a second capacitor and the primary winding of a transformer. The secondary winding of the transformer is connected to apply an ignition signal to the flash tube. When the flash trigger switch is switched to a conductive state, the second capacitor discharges through the flash trigger switch and the primary of the transformer. The voltage pulse induced in the secondary of the transformer is applied to the flash tube to turn the flash tube on.
The control electrode of the flash termination switch is also connected to the flash trigger switch. When the flash trigger switch is switched to a conductive state, the flash termination switch is likewise switched to a conductive state.
One problem which can occur in circuits of this type is damage to or destruction of the flash trigger switch, the flash termination switch, or both, caused by large (about 7 volts) negative gate-to-cathode voltages. Protective circuitry can be provided to overcome this problem, but such protective circuitry ordinarily involves use of several additional components, thereby increasing the cost of the electronic flash apparatus.